Flexible printed circuits (FPC) have features such as flexibility and bending properties, and are widely used to incorporate circuits into complex mechanisms in various electronic devices such as mobile phones, video cameras, and laptops, which are rapidly becoming smaller, lighter and thinner.
The FPC is formed of a copper clad laminate (CCL) having a circuit formed by etching process and a cover coat material. The cover coat material is generally selected from a cover lay (CL) film, a photosensitive ink, a photosensitive film (photosensitive cover lay film) or the like. Among these, in a case of being used as a surface protection material for the FPC, a cover lay film obtained by applying an adhesive to a molded body (support) such as a polyimide film is often used from the viewpoint of ease of handling, durability, and insulation reliability in the thickness direction. The following mainly describes the FPC, but it can also be used for semiconductor package applications (semiconductor PKG applications). In addition to the copper clad laminate having a circuit formed by etching process as described above, a substrate on which a circuit is formed by printing using a paste-like nano-ink containing conductive fine particles such as silver particles and copper particles can also be used as a substrate for the FPC. The following mainly describes the CCL, but a substrate on which a circuit is formed by printing using a paste-like nano-ink containing conductive fine particles such as silver particles and copper particles is also included.
In a case of using a cover lay film including a polyimide layer for the FPC, it is a common method to provide an opening portion in the cover lay film at a part corresponding to a terminal portion of the circuit of the CCL, to position the opening portion and the corresponding terminal portion, and then to thermally compress the two with a heat press. However, in the above method, the size of the opening portion provided is limited, it is difficult to make a small hole of 1 mm or less in the micron unit, and the alignment during attaching is often performed manually, thus causing a poor position accuracy and also a poor workability of the laminating.
Thus, in recent years, a solder resist has come to be used as a surface protection material for the FPC, and particularly, a solder resist having a photosensitive function is preferably used when fine processing is required.
In the FPC having a solder resist layer, when the solder resist layer becomes thinner due to the miniaturization and thinning of the electronic devices, the CCL circuit is susceptible to electromagnetic waves. Therefore, an electromagnetic wave shield may be provided. However, on the other hand, when a shield layer made of a conductive material such as an electromagnetic wave shield is provided, upon applying a voltage, ions (metal ions) are eluted from a metal forming the CCL circuit and the migration phenomenon of moving in the solder resist layer, which is an insulator, is likely to occur in the thickness direction (hereinafter, also referred to as “Z axis direction”) of the FPC. The metal ions eluted in the solder resist layer come in contact with each other to cause a short circuit, leading to system failure or the like.
Since polyimide is excellent in insulation as described above, the migration phenomenon in the thickness direction hardly occurs in the case of using the cover lay film including a polyimide layer, but the migration phenomenon in the thickness direction is likely to occur in the case of using the solder resist.
As a technique of improving the insulation properties of the solder resist layer in the thickness direction, for example, Patent Literature 1 proposes a photosensitive resin composition containing:
(A) a carboxyl group-containing photosensitive resin obtained by adding
d) a polybasic acid anhydride to a reaction product of a) an epoxy resin having two or more epoxy groups in one molecule, b) at least one type of fatty acid having 10 or more carbon atoms per carboxyl group and c) an ethylenically unsaturated group-containing carboxylic acid; (B) a photopolymerization initiator; (C) a (meth)acrylate monomer having a hydrophobic skeleton; (D) an epoxy compound; and (E) an inorganic cation exchanger. According to the photosensitive resin composition described in Patent Literature 1, it is supposed that an insulating film excellent in insulation reliability in the thickness direction is obtained.